Sound recording and reproducing system



Nov. 22, 1932. c. R. HANNA SOUND RECORDING AND REPODUCING SYSTEM Filed Nov. 25, 1928 lNvENToR Clmi'on R. Hanna TTORNEY Patented Nov. 22, 1932 UNITED STATES PATENT- oFF-Ice CLINTON R. HANNA, 0F WILXINSB-URG, PENNSYLVANIA, ASSIGNOR TO 'WESTINGHOUSE ELECTRIC @c MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA- soUND RCORDING 'AND REPRODUCING SYSTEM Application led November 23, 1928. Serial No. 321,365.

The invention relates to methods of and means for recording and reproducing sound and particularly to photographic methods and means for this purpose in which the intensity'of the sound is represented by variations in the'amount of light delivered to the record.

All soundrecording-and-reproducing devices introduce more or less extraneous noises, including ground tones. When the sound to be recorded is fainter than that to which the apparatus is ordinarily adjusted, as, for eX ample, when it is a diminuendo passage in music, the distinction between'the sound and the ground tone is less, and the satisfactory recording and reproduction of such sounds is thereby rendered difficult.

It is an object of my invention to provide a means for satisfactorily recording and reproducing sounds, evenwhen the program includes both loud sounds and faint sounds.

vIt is a further object of my invention to provide a variable degree of amplification between the sound-responsive device and the recording instrument, whereby a greater degree of amplification may be utilized for' faint sounds. l

It is a further object of\my invention to provide a ,feature in the record by means of which the degree of amplification utilized in reproduction may be so controlled that sounds which were recorded with a high degree of amplification may be reproduced with a small degree of amplification.

It is a further object of my invention to 'provide automatic means whereby the degree of amplification between the microphone and the recorder maybe made to correspond to the average intensity of the sound and also automatic means whereby the degree of amplification between the record-controlled device andthe soundproducing device may be automatically adjusted' in accordance with the above-mentioned feature of the record.

It is a further "object of my invention to provide means whereby the average degree of darkening of the negative may be controlled in accordance with the average in-` tensity ofthe sound. The excess or deficiency v of darkening, at anylpoint of the record, cor- DEC l 2 i539 responding to the momentary sound pressure, will, in the case of the area type of record, be manifest as 'peaks or valleys atv one edge of .the darkened area.

In the case of a photographic/record of the type in whichmot area but density varies with the sound, the momentary changes corresponding to'instantaneous sound values will be manifest as bands or lines across the recl ord, denser or thinner than the average. In a negative of this type, the average density changes as the average width of the darkened area changes in the record of the firstdescribed type. y

Changes in the degree of darkening, whether they are changes of density or of area of the darkened portion, can represent the sound values, and thev average darkening can represent the vaverage intensity of the sound in `either type of record.

Obviously, both the average character and the variations from point to point in the.

negative are represented by corresponding opposite characteristics in the positive. Where the average darkening of the negative is greatest, the positive will' show a greater average degree of clearness'. -In the case of the area-typeof record, this means that the clear area in the vpositive will lbe greater at such places. In the case of the variable-density type of record, this means that the' positive will, on the average, be more transparent there.

Itis a further object of my invention to so correlate the change in average darkening with the .change in amplification that the individual maxima of darkening will be vof a more nearly constant value. l A

It is a further object of my invention to provide a means whereby the average value of the rectified current from a, microphone may be indicated byl a bias upon the grid ofthe res onding ig. 3 is a diagram of the circuits and ap- I paratus used in the reor'dingu system, and

. ing picture and,

. rectangular pictures V; any desired point,

placed,

Fig. 4 is a diagram of the circuits and apparatus used in the reproducing system.

1anV the negative shown in Fig. 1, the frames 1 vrepresent the pictures of an ordinary movrecord. The rectangle 2 represents the guidel through which the record must pass while recording sound. The window 3, which is shown. in Fig. 3, also represents a slit through which the record is illuminated to a greater or less extent,.corre sponding to the sound.

The positive record shown in Fig. 2 comprises corresponding -parts, including the 5. A gate 6-and a Windovv 7 cooperate with the sound record 8. The sound record 8, represented by the clear area in Fig. 2, corresponds to the darkened ed area 9 in In the apparatus shown in Fig. 3, a microphone 11 supplies current in the output circuit 12 which includes the primary of a transarea in the negative represented by the shad- P former 13 and the primary of a transformer 14 inparallel thereto.

The secondary of the transformer 13 is connected to the id of. avacuum tube 15 through a resistor 16 and a biasing battery 17, to the lament of the same tube. The tube 15 is connected to an amplifier 18,

vthe connection being of a character which causes the tube 15 to amplify to a degree determined by the averagey potential upon its grid.l One well-known circuit for producing this'result is illustrated. It includesa resistor 21 in the plate circuit of the tube 15 and a condenser 22 .by means .of which the end of said resistor nearest the anode of the ,*tube is Yconnected to the input of the amplifier. 1,8.

The usual plate battery 23 is inserted, at in the plate circuit. If as shown, at the filament end of vthe l resistor21, -the 'same plate battery may also serve as the B-battery for the amplifier- 18.

The output of the amplifier 18 is connected,

I .preferably through a transformer y24, to the vibrating element of an oscillograph 25. A light source 26, with which are associated a diaphragm 27 andan'ptical system, reprez sharp straight sented by the-lens 28, illuminates the mirror of theoscillograph.A The light reflected from the mirror is received .upon the slit 3 and illuminates more or less of the film, according to the position of the mirror. The diaphragm 27 lis preferably made to present a edge to the light in order that The transformer 14 supplies the input of l' a vacuum tube 30 which is equipped with a grid condenser 31 and grid leak 32, causing the tube 30 to act as a rectifier. A low-passy filter is inserted in the plate circuit of the tube 30. This filter includes an inductor 33 and a pair of condensers 34. The B-battery 35 is shown as outside of this filter but it may be inserted at any convenient point in the plate circuit.

The resistor 16 is connected across the late circuit of the tube- 30 outside of the lter. If desired, an adjustable resistor 36 may be included inthe shunt connect-ion. The"v resistor 39 of a potentiometer isv connected in parallel to the resistors 16 and 36. Between the slider ofthe potentiometer and one end thereof, a condenser 41 is connected.

The output circuit of the potentiometer is connected to the secondary of the transformer 24 in series with the oscillograph 25. The condenser 41, the transformer secondary and the vibrator of the oscillograph thus form a circuit which, preferably, is of `small iinedance to currents of audio frequency.

In Fig.I 4, the light source `45 is concentrated by any suitable optical system, represented by the -lens 46, upon a sound record which is made by printing a positive, like that shown* in Fig. 2, from a negative, like that shown in Fig. 1. The light which traverses the sound record is received in a photo-cell 47 which is supplied with current from a battery 48. The circuit of the battery 48 includes the primary of a transformer' 49, a resistor 51 and the photo-cell 47 in series.

A resistor 52 .is `connected across the secondary of the transformer 49 in a loop circuit which includes the primary of a transformer 53, through which the link circuit is connected to the grid circuitof a vacuum tube 54, connected to /an amplifier 55, preferably in the same Way as the tube 15 to its amplifier 18. The output of the amplifier 55 supplies ay` loud speaker 56 or other sound-reproducing device.

In shunt to the resistor 51 is a condenser 57 which is connected to the grid and/filament of the tube 54. 'A C-batter 58 is included in the connection to provide a the tube 54. the operatipn of the device, the sound to corded is rceived upon the microphone Currents produced thereby in the circuit 11'. 12 are Vtransmitted by the transformer 13 to the grid and filament of the tube 15 and amplified thereby to a degree determined by the average potential upon the grid 15.

Currents in the circuit 12 produce varying ias upon the grid of potentials upon the grid :of the rectifier tube Because of the condenser 31 and the resistor 32, the changing potentials cause,` a charge to accumulate upon vthe grid of the tube 30 and thus vary its potential in a way which corresponds to the average value of the amplitude of the changing potentials.

This action is well understood, being strictly analogous to the action of the usual gridcondenser-grid-leak type of detector. The difference, however, is that the oscillations in the potential are much slower than in the usual detector tube because the input from the circuit 12 is of audio frequency. The detector or rectifier tube 30 delivers a current to its output circuit which has a very small audio-frequency component and the main compound of which varies in accordance with the average intensity of the sound at the microphone 11. In order that momentary abrupt changes of intensity or the instantaneous changes, corresponding to the sound pressure, shall not produce corresponding abrupt. current changes in the output of the tube 30, the filter 33-34 is 1 provided. Preferably, this filter is of a low-pass type, greatly attenuating audio frequencies.-

Frequencies of the order of one or two cycles per second, 0r even, as low as a cycle in one or two seconds are transmitted by the filter and produce corresponding changes in the current in the resistors 16 and 39. The correct division of the current between'these two resistors may be adjusted by adjusting the resistor 36.- A further adjustment of the lrelation between the edect, at the oscillograph, of the steady .or slowly-changing current through the resistor 16 and of the potential changes at audio-frequency transmitted by the transformer 13 is obtained by adjustment of the potentiometer 39.

When the current throughthe resistors 16 and 36 increases, the'potential drop across the resistor 16 increases. The left-hand e'nd of the resistor 16, therefore, becomes more negative, theimpedance of thetube 15 increases, the proportion of all the impedance in the circuit of the battery 23 to the impedance of the resistor 21, therefore, increases, and the potential changes delivered across the condenser 22 are, therefore, of smaller amplitude.

It will thus be seen that a change in the average potential across the resistor 16 results in 'a change in the average amplification performed in the tube 15. If this change, as the apparatus is first set u is in the opposite direction to that desired), a reversal of the connections from the tube 3,0 to the ends of the resistor 16 will reverse the direction of change in the degree of amplification.

Faint sounds produce small changes upon the grid of the tube 30, causing only a small negative charge `to accumulate thereon and so diminish the current in the plate circuit of the tube 30 only slightly from the value thereof corresponding to silence. Loud sounds produce large changes of potential upon the grid 30 and, consequently, diminishthe cury rent through the resistor 16 by a large amount.

Therefore, faint soundsI result in a high degree of amplification, and loud soun s are amplified to a smal-ler degree.

are shown as of greater height, to represent f the louder sounds, but they are not of as great a relative height as'they vwould be if the amplification corresponding to loud sounds were-not diminished as explained.

In addition to the change in amplification, there is a change in the zero point from which the waves in the record begin. This is accomplished by the steady current through the output of the potentiometer 39. The potentiometer delivers a steady current through the oscillograph 25 which is superposed upon the audio-frequency current introduced through the transformer 24. The effect of the steady current through the oscillograph is to permanently deflect the mirror and thus bring at least a portion of the slit 3 into the region of permanent illumination, or if the permanent deflection be in the opposite direction, a region of permanent cause, in addition to the change in amplitude l during a crescendo or diminuendo passage, there are, in any program, momentary louder sounds and moments of silence which are not removed by the slow correcting action of the tube 30.

It is not desirable that they should be removed. The reproduction ought to give these effects with the others. The straight line along the top of the darkl peaks in Fig. 1, or the top of the light-colored peaks in Fig. 2, represents this characteristic of the record of a sound of gradually changing intensity, unaccompanied lby any sudden changes to louder or fainter sounds.

The film, having been exposed in the apparatus shown in Fig. 3, is developed and printed, thus producing a record like that shown in Fig. 2. Such record is then used in an apparatus like that sho-wn in, Fig. 4.

l/Vhen a portion of the record film corresponding to loud sounds, for example, that in the upper part of Fig..2 is passing in front of the optical system 4G, the photo-cell 47 1s illuminated only by the passage of the peaks in front of the' slit. There -is no area unbroken by valleys passing before the slit. Therefore', the cell is only moderately illuminatediI I s The average current through the circuit of thebattery 48 is, therefore, small, the potential across the resistor 51, is small and the grid of the tube 54 is, therefore, less negative than under average conditions.

YVhen the grid is less negative ,the amplification is greater. Therefore, the sounds which are` recorded with a relatively small amplification when exposing the film are reproduced with a relatively large amplification.

The sound record represented near the: hottom of Fig. 2, which corresponds to the record of faint sounds, presents a relatively wide band of clear film. It, therefore, causes the photo-cell 47 to .be illuminated strongly.

` as compared with the ground tone. The rec-- This results in a more negative grid at the tube 54 and, therefore, a smaller'degree of amplification'. Thus, the sounds which were:

recorded with a' large degree of amplification are reproduced with a small degree of amplification. For this reason, the sounds are not over-emphasized in the reproduced program but appear as faint sounds.

It will be seen that the effect of ground tone, whether due to film noises or to noises Ain the amplifiers, is largely avoided by this' procedure. Loud sounds may -be recorded L upon the film, without difficulty from ground tone because the latter is negligible, as com: pared with them. Faint sounds, having been amplified vto a greater degree, are 4then large,

ord ground tone will, therefore, not inter'- fere with the reproductionof the amplified sounds. l/Vhen reproducing from the record, a high degree of amplification is not employed, even for the faint sounds, and, there- "fore, any danger of magnifying the ground tone, even when reproducing falnt sounds, 1s`

avoided. l

The means described for impressing a slow- '1y changing potential upon the grid of the tube 15 in the recording device vor upon the grid of the tube 54 in the reproducing dejustments guided either by ammet'erreadyice are automatic. Obviouslyl manual adings or by the operators estimate of the average loudness maybe made to give the same results.

Byreversing the connections from the grid and ilamentto the resistor either at the tube 15 or the tube 54, the negative, instead of the positive, may be used for reproduction. '.lhe system, with the connections then reversed and the oscillograph adjusted for the zero current position near the sprocket-holes instead of near the picture-record, as shown in Figure), will produce a negative in which the crests near the edge of the film will f ollow approximately a straight line. The shift of the mid-point of the successive undulations with changing amplification, being in the opposite sense when the connections to resistor 39 are reversed, the average degree ofdarlf` emng of the negatlve, as a whole, now decreases instead of increasing with faint,

Afier includin a vacuum tube means for imn 7 l pressing a potential varying at sound fre-4 quency upon the grid thereof, means controlled by the intensity of the sound forimpressing a slowly varying potential upon said grid, a recorder including a member moved by the output'of said amplifier and means responsive to the said slowly varying potential for controlling the mean position of said member.

2. In a systemfor reproducing sound from .a photographic sound-record, a photo-cell the illumination of whi'ch is controlled by said record, a circuit in which the current is controlled by said photo-cell, a Vacuumtube, l

means for impressing potential corresponding to the sound-frequency variations of current in said circuit upon the grid of said vacuum tube, and means for impressing a 'biasing potential upon said grid which Varies in'accordance with the variation in the avera ge'illumination of 'said photo-cell.

3. A photographic sound .record having variations in darkening corresponding to audiofrequency, the amplitude of said variations bearing a ratio to the .amplitude of the correspondingsound, which ratio is different at different parts of the record, and-the average amount ofdarkening corresponding to said ratio at each part of the record, whereby the the 'average sound.'

gram which consists intranslating the sound into electrical 'pulses amplifying said pulses to produce amplified perimposing sald current pulses on a relatively steady current', controllingthe'deg'ree of 4. The method of reproducing ais-ound pro-' amplification ofsaid pulses'to `cause a greater degree of amplification for faint sounds than for loud sounds, so modifying the' amplitud'e of said relatively steady currentthat the sum of said 'steady current ,and the maximum average degree'of darkening is correlated to intensity of the corresponding current pulses and suwhich consists in translating the sound into electrical impulses, amplifying said impulses to produce amplified current pulses and superimposing said current pulses on a relatively Steady current, controlling the degree of amplification of said pulses to causea greater degree of amplification for faint sounds than for loud sounds, so modifying the amplitude of said relatively steady current that the sum of said steady current and the maximum value of said current pulses is an approximately constant value, and forming a record of the variable arca type in accordance with said sum.

6. A translating system comprising means for impressing the record of a physical phenomenon on a permanent medium, said record being of the type in which the darkened area of the negative accords With the instanta-neous values of said phenomenon and in which the area has peaks corresponding to the intensity of said phenomenon, and means responsive to said phenomenon for so varying the mean position of that boundaryline of tbe record Which is defined by said peaks that the tops of said peaks attain substantially a common level.

7. In translating apparatus of the type if:-

fcorporating means for transforming a me chanical disturbance into an electrical disturbance and means for impressing a record of said electrical disturbance on a permanent medium, said electrical disturbance being microscopic in character, said record being of the ltype in which the darkened area in the negative accords with the instantaneous value of said electrical disturbance and in which said area has peaks corresponding to the intensity of said electrical disturbance, means for introducing a microscopic variation in said electrical disturbance, said electrical disturbance and said microscopic variation being so related that the tops of said peaks are at substantially a constant level.

8. Sound-recording apparatus comprising means for transforming sound energy into electrical energy, means for varying the amplitude of the electrical energy of said transformed sound, means for impressing a rey cording of said electrical energy on a permanent medium, said record being of the type in which the darkened area ofthe negative accords with the instantaneous values of said electrical energy and in which the area ha-s peaks corresponding to the amplitude of said electrical energy, and means responsive to said amplitude for varying the mean position of that boundaryline of the record Wliich is defined by said peaks.

9. Sound-recording apparatus comprising means for transforming sound energy into electrical energy, means for impressing a record of said electrical energy on a permanent medium, and means for varying the zero boundary of said record in accordance With the amplitude of said impressed sound energy.

l0. A photographic sound record in Which the negative has variations in darkening corresponding to audio-frequency, the amplitude of said variations bearing av ratio to the amplitude of the corresponding sound, said ratio being larger for small sound amplitudes than for large sound amplitudes, and the average amount of darkening increasing With the increase in said ratio.

1l. Apparatus for reproducing sound from a positive corresponding to the negative described in claim l0 comprising a photo-cell, an amplifier, means for passing light through said positive onto said photo-cell, and means for decreasing the amplifying ability of said amplifier lin response to an increase of the average amount of light falling on said photocell.

l2. A sound record of the variable density type comprising regions of. darkness interspaced with background regions of lesser darkness characterized by vthat the average intensity of the background regions varies in accordance With the intensity of the sound recorded.

In testimony whereof, I have hereunto subscribed my name this 14th day of November CLINTON R. I-IANNA. 

